1. Field of the Invention
The present invention relates to the production of metal oxide aerogels, and particularly to a method for making transparent, monolithic metal oxide aerogels of varying densities.
2. Background of the Invention
Aerogels are a special class of open-cell foams derived from the supercritical drying of highly cross-linked inorganic or organic gels. These materials have ultrafine pore sizes of less than 1000.ANG., continuous porosity, high surface areas of typically 400-1000 m.sup.2 /g, and a microstructure composed of interconnected colloidal-like particles or polymer chains with characteristic diameters of 100.ANG.. This microstructure is responsible for the unusual optical, acoustic, thermal, and mechanical properties of aerogels.
Silica aerogels are the most extensively described aerogel materials in the scientific and patent literature. Aerogels of transition metal oxides, in particular, are not as well described, and these aerogels are expected to possess properties that are not possible with silica aerogels due to the presence of the transition metal. The new characteristics of the aerogels will produce interesting new materials for optical, magnetic, and catalytic applications.
The first aerogels were translucent pieces of porous silica glass made by S. S. Kistler (U.S. Pat. No. 2,249,767). Kistler's aerogels are prepared by forming silica hydrogels, which are exchanged with alcohol and dried. The alcohol is supercritically extracted in the drying process, and the resulting aerogel has a density of about 0.05 g/cm.sup.3. Kistler's process is time-consuming and laborious, and subsequent advances in the art have reduced the processing time and increased the quality of aerogels.
Other related art discusses the production of metal oxide aerogels other than silica aerogels. Teichner et al., in Advances in Colloid and Interface Science 5:245-273 (1976), provides a general discussion of metal oxide aerogels, including oxides of silicon, aluminum, titanium, zirconium, magnesium, nickel, copper, and molybdenum. Lynch (U.S. Pat. No. 3,977,993) discusses a modified Kistler method for making metal oxide aerogels. These aerogels are made by preparing a hydrogel, exchanging the water in the gel with an organic solvent, and then supercritically extracting the organic solvent. The Lynch patent does not discuss the peculiar problems in using different metals, such as tantalum, and the process conditions necessary to ensure that the resulting aerogels form large, transparent, intact solids.
European Pat. No. 0382310 by Enichem discusses a process for preparing monoliths of metal oxide aerogels. The process comprises an acidic hydrolysis of a metal alkoxide, the gelation of the resulting colloidal solution, and the supercritical drying of the gel. The patent recognizes the difficulty in obtaining monolithic aerogels with metals other than silicon. The patent addresses the problem by adding a volatile powder of a metal oxide to the colloidal solution at the end of hydrolysis, before gelation.
European Pat. No. 0186149 by Stauffer Chemical Co. describes the preparation of non-aged, inorganic oxide-containing aerogels. The method comprises the steps of dissolving the alkoxide in a solvent, optionally adding a catalytic amount of a base or acid, and hydrolyzing the metal compound to produce a gel. At least a stoichiometric amount of water is used in this process for the hydrolysis reaction. The solvent in the gel is exchanged with an extraction fluid, and the fluid in the gel is supercritically extracted to form an aerogel. The patent describes the preparation of amorphous, granular metal oxide aerogels, rather than in monolithic forms, and does not mention transparency.
Although these related patents discuss the formulation of metal oxide aerogels, they do not describe methods to overcome the special problems of producing transparent, monolithic aerogels with variable densities, surface areas, and porosities for materials that are not exclusively silicon oxide. Such materials include, for example, the metal oxide aerogels of tantalum, niobium, tungsten, molybdenum, hafnium, zirconium, titanium, vanadium, chromium, rhenium, the lanthanide metals (Ce-Lu), and may include boron, aluminum, gallium, indium, silicon, germanium, tin, lead, or any mixture of these metals. The present invention recognizes and addresses the need for a method that produces transparent, monolithic metal oxide aerogels of varying densities.